It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473, 4,115,128, 4,173,470 and 4,550,069. These include alkali-soluble, film forming polyvinyl phenol or novolak resins together with light-sensitive materials (sensitizers), usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The novolak or polyvinyl phenol resin component of these photoresist formulations are soluble in aqueous alkaline solutions, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to these film forming resins. Upon exposure to actinic radiation of selected areas of the substrate on which the photoresist composition has been coated, the sensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered much more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is exposed to an alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
An important group of photosensitizers are the condensation products of 1,2-naphthoquinonediazide-4-sulfonic acid or 1,2-naphthoquinonediazide-5-sulfonic acid and polyols. These compounds tend to exhibit superior photospeed and contrast in the mid-UV region of the light spectrum when formulated in photoresist compositions.
The trend in microlithography during the past few years has been an accelerated drive towards smaller geometries. The efforts to achieve these shrinking design rules require intensive efforts by both exposure tool and photoresist manufacturers. Therefore, there is an increasing demand for UV-2 (DEEP-UV) and UV-3 (I-LINE) sensitive photoresist systems. Novolak-containing photoresists are still workable for g-line and i-line type resists. 2,1,5-diazonaphthoquinone sulfonate esters of trihydroxy benzophenones are generally used as photoactive compounds (PAC) for broad band or g-line resists. On the other hand 2,1,4-diazonaphthoquinone esters are more generally suitable for the i-line (365 nm) region of the spectrum.
Synthesis of such a 2,1,5-diazonaphthoquinone ester is normally conducted in a solvent system such as: N-methyl pyrollidone (NMP), acetone, acetonitrile or a mixed solvent system. A base is generally used as a catalyst and also as an acid acceptor. Common organic bases used include pyridine, triethylamine, N-methyl morpholine, dimethyl amino pyridine and mixtures thereof. The processes utilizing such solvent/base systems provide diazonaphthoquinone esters of consistently good quality. However, providing similar quality and consistent particle size for the diazo ester becomes extremely difficult if a similar process is utilized for making a 2,1,4-diazo ester or a 2,1,4-/2,1,5- mixed diazo ester. In addition, making a 2,1,4-diazo ester or a 2,1,4-/2,1,5- mixed diazo ester becomes even more difficult when utilizing a tetrahydroxybenzophenone. The PAC may be precipitated from the reaction solvent after the condensation reaction by adding the reaction mixture to either an aqueous organic solvent, such as methanol or to an aqueous systems that contain an acid, as is well known in the prior art.